The hematopoietic niche is a complex structure of multiple cell types and extra-cellular matrix proteins. In a well-orchestrated manner, elements of the niche interact together and with hematopoietic stem cells (HSC) to maintain HSC selfrenewal potential. HSC maintenance within the bone marrow (BM) is associated with the health of cellular elements of the niche including endothelial cells, osteoblasts, and other hematopoietic cells such as megakaryocytes. Our published work demonstrates that immature osteoblasts mediate a robust in vitro hematopoiesis enhancing activity and that megakaryocytes enhance osteoblast proliferation and inhibit their differentiation. Megakaryocytes have been implicated in both regulating HSC function and maintaining the competence of the niche after radiation through specialized interactions with osteoblasts that augment their enhancement of HSC function. Recently, a unique population of CD45+F4/80+ macrophages known as osteomacs (OM) was recognized in the niche. We detected these cells in neonatal calvarial cell (NCC) preparations and recently published that OM are critical for the osteoblast-mediated hematopoiesis enhancing activity. Megakaryocytes stimulate NCC-derived OM as well as OM from adult mice and significantly enhance their in vitro expansion and function. Interestingly, extensive flow cytometric characterization of OM revealed that OM are phenotypically distinct from BM-derived macrophages and that the later cannot functionally substitute for OM to drive the osteoblast-mediated hematopoiesis enhancing activity. Our studies further suggest that OM are important for the competence of the hematopoietic niche. We hypothesize that maintenance of HSC function and the competence of the hematopoietic niche are dependent on cellular interactions and molecular cross talk between osteoblast, OM and megakaryocytes. Our hypothesis will be examined by investigating the following three aims: 1) Investigate if OM are transplantable and whether loss of megakaryocytes disrupts the emergence of OM and negatively impacts HSC function and niche competence. 2) Identify differences between OM and BM-derived macrophages that make OM a unique niche component and define, at the molecular level, how OM and megakaryocytes promote the maintenance of HSC function. 3) Define the spatial relationship between HSC, osteoblasts, OM, and megakaryocytes in the intact niche of young and old mice and in the perturbed microenvironment following marrow conditioning. The significance of these studies is that they will define and explain how the interplay between four cellular components of the BM regulate HSC function and the competence of the niche. The novelty derives from the potential of these studies to establish, for the first time, a unique group of cells, namely OM, as primary targets of the megakaryocyte-mediated HSC promoting activity in the niche. Our premise that OM are central to HSC and niche functional properties is both paradigm shifting in our understanding of the close interactions between HSC and the niche and is also an unexplored pathway critical to the maintenance of hematopoiesis.